# File lib/em/protocols/smtpserver.rb, line 441
      def process_mail_from sender
        if (@@parms[:starttls]==:required and !@state.include?(:starttls))
          send_data "550 This server requires STARTTLS before MAIL FROM\r\n"
        elsif (@@parms[:auth]==:required and !@state.include?(:auth))
          send_data "550 This server requires authentication before MAIL FROM\r\n"
        elsif @state.include?(:mail_from)
          send_data "503 MAIL already given\r\n"
        else
          unless receive_sender sender
            send_data "550 sender is unacceptable\r\n"
          else
            send_data "250 Ok\r\n"
            @state << :mail_from
          end
        end
      end